Currently, it has become possible to commercialize an optical disk having a recording capacity of about 50 GB even for a consumer use by a Blu-ray Disc™ standard using a blue-violet semiconductor laser. Hereafter, it is desired to increase the capacity to the same extent of an HDD (Hard Disk Drive) capacity of 100 GB to 1 TB even for an optical disk.
However, in order to realize such a very high density in an optical disk, a density increasing technology by a new method that is different from a density increasing technology by wavelength shortening and objective lens NA increase is needed.
While a study regarding a storage technology of the next generation is being made, a hologram recording technology of recording digital information utilizing holography is attracting attention.
The hologram recording technology is a technology of recording information in a recording medium by superimposing signal light having information of pagedata two-dimensionally modulated by a spatial light modulator with reference light inside a recording medium and generating refractive index modulation inside the recording medium by an interference fringe pattern generated at the time.
When playing back the information, when the recording medium is irradiated with the reference light used during recording, a hologram recorded on the recording medium acts like a diffraction grating and generates diffracted light. The diffracted light is played back as the same light as the recorded signal light including phase information.
The played-back signal light is two-dimensionally detected at a high speed using a photodetector such as a CMOS or a CCD. In this way, the hologram recording technology makes it possible to record two-dimensional information in an optical recording medium at once by one hologram and reproduce the information further, and since two or more pieces of pagedata can be overwritten at a certain location of the recording medium, a large capacity of information can be recorded and played back at a high speed.
A configuration of a hologram recording and playback device is described in Patent Literature 1 for example. The literature describes that “a shape of a recording medium 1 has a disk shape for example, and is fixed to a spindle motor 200 by a clamp mechanism, and by rotational drive of the spindle motor 200, an irradiation position of a coherent beam on the recording medium can be moved in a tangential direction. The spindle motor 200 is fixed to a thread motor 201, and by rotational feed by the thread motor 201, the irradiation position of the coherent beam on the recording medium can be moved also in a radial direction.”